A new look at Greenland's ice sheet and the bedrock below

The bedrock hidden beneath the thick ice sheets covering Greenland and Antarctica has intrigued researchers for years. Scientists are interested in how the shape of this hidden terrain affects how ice moves — a key factor in making predictions about the future of these massive ice reservoirs and their contribution to sea level rise in a changing climate.

NASA has been monitoring Antarctic and Arctic ice since 2009 with the Operation IceBridge airborne mission. Although the primary objective is to continue the data record of ice sheet surface elevation changes from NASA’s Ice, Cloud and Land Elevation Satellite, or ICESat, which stopped functioning in 2009, IceBridge is also gathering data on other aspects of polar ice from snow on top to the bedrock below. One radar instrument on these flights that is currently headed to Antarctica for another year of observations is revealing insights about the bedrock hidden beneath the ice sheet.

IceBridge is carrying a suite of radar instruments designed, built and operated by scientists, engineers and university students with the Center for the Remote Sensing of Ice Sheets (CReSIS), a National Science Foundation-funded center based at the University of Kansas.

This bedrock-mapping radar is known as the Multichannel Coherent Radar Depth Sounder or MCoRDS. MCoRDS measures ice thickness and maps sub-glacial rock by sending radar waves down through thick polar ice. This ice-penetrating radar is the result of efforts that started with a collaboration between NASA and the National Science Foundation 20 years ago.

In the early 1980s, researchers started showing interest in using radar to measure ice thickness and map sub-ice rock. Among the interested was NSF’s Office of Polar Programs, which provided the initial funding for a thickness-measuring instrument. “We were given one year to prove it would work,” said Prasad Gogineni, scientist at CReSIS. CReSIS researchers used that funding to build their first radar depth sounder, which started flying aboard NASA aircraft in 1993. Over the years, CReSIS has built a number of instruments – each more advanced that the last – leading to the radar IceBridge relies on today.

Through the Ice

One of the biggest obstacles faced when building an ice-penetrating instrument like MCoRDS is the nature of radar itself. Radar works by sending out radio waves and timing how long it takes for them to reflect back. Radio waves travel through air virtually unimpeded, but materials like metal, rock and water act almost as mirrors.

Ice, on the other hand reacts differently depending on the radar’s frequency. It reflects high-frequency radio waves, but despite being solid, lower frequency radar can pass through ice to some degree. This is why MCoRDS uses a relatively low frequency—between 120 and 240 MHz. This allows the instrument to detect the ice surface, internal layers of the ice and the bedrock below. “To sound the bottom of ice you have to use a lower frequency,” said John Paden, CReSIS scientist. “Too high a frequency and signal will be lost in the ice.”

These radio waves are sent out in rapid pulses through an array of downward-pointing antennas mounted beneath the aircraft. This array of multiple antennas, up to 15 on NASA’s P-3, allows researchers to survey a larger area and record several signals at once to get a clearer picture.

A MCoRDS’s underwing radar antenna arrays mounted beneath the wing of NASA’s P-3 aircraft at McMurdo Station’s airfield. MCoRDS uses arrays beneath the wings and fuselage of the plane, giving a total of 15 antennas that can send and receive signals used to map polar ice and bedrock. Image Credit: NASA/George Hale

The 15 element array is the largest ever flown on the P-3 and is was built as part of a joint effort between NASA, the University of Kansas and private industry. The design and construction of this array, much of which was done by University of Kansas undergraduate and graduate students, took about six months.

Radar pulses travel down to the surface, through the ice to bedrock below and back up through the ice to MCoRDS’s array, where they are routed to the instrument’s receiver and recorded on solid state drives aboard the aircraft.

Each survey flight yields a great deal of data, often as much as two terabytes, that then needs to be downloaded, archived and backed up. The computing infrastructure needed to handle this data is managed by people from the University of Indiana, which is also a partnering organization in CReSIS. During each campaign, University of Indiana personnel support the mission by staying up through the night to ensure that the data collected each day is successfully stored and backed up.

Processing Pictures

After returning from an IceBridge campaign, CReSIS researchers spend months processing the archived data to build a detailed view of ice sheets and bedrock. First, researchers tease out the return signals from the ice surface and bed. Because thick ice attenuates, or weakens, radar so researchers need to filter the data to pick out the weak return signal from the bed, which would otherwise be drowned out by the much stronger surface return and any noise in the data.

After finding the ice surface and bedrock, researchers use something called synthetic aperture radar processing. This combines many readings from a radar antenna as it moves over the surface researchers can create a large simulated array. “You can make an aperture one kilometer long by moving the radar one kilometer,” said Paden. As with camera lenses, bigger is better, and a larger array lets researchers see more detail.

This sort of processing yields a detailed, but narrow, swath of the ice and sub-ice terrain for each antenna. Building a wider view is more complicated than just combining these separate signals. Although MCoRDS records signals coming back from the left and right of the plane’s flight path, it cannot determine which side the signals are coming from. To overcome this, CReSIS researchers use something known as tomography, a technique that uses specialized computer software to calculate the position and distance of signals returned from the bedrock. Once researchers can tell where terrain features are relative to the array they can combine the several channels to build a swath of terrain data useful for creating three-dimensional representations of the bedrock.

The Road Ahead

These terrain data are helping scientists better understand what’s under ice sheets. In the past year, researchers have produced new maps of Greenland’s and Antarctica’s bedrock and discovered a large and previously undetected canyon under Greenland’s ice sheet. Better information on sub-ice terrain will help researchers develop the next-generation ice sheet models needed to project future changes to glaciers, and better understand the flow of water at ice sheet bases.

As IceBridge continues to add to the record of sub-ice terrain measurements through its surveys over Greenland and Antarctica, scientists, engineers and students at CReSIS will keep making more advances. Improvements such as larger antenna arrays and improved data-processing techniques promise to make radar depth sounding even more effective. And in the future, uninhabited aerial vehicles like NASA’s Global Hawk, could greatly increase the amount of terrain that can be covered from the air.

Exactly what the future holds remains to be seen, but researchers have made great strides in probing polar ice thanks to a decision made by one person, former NASA program manager Bob Thomas, who provided Gogineni and his team the opportunity to prove that their instrument worked and funding from NASA’s PARCA initiative. “Without that, we would not have a depth sounder and imager program at KU,” said Gogineni. “It took both agencies to make it to the stage we are today.”

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I wonder what causes the large-scale layering of the ice above the bedrock? Wouldn’t it be cool if that was a record of all the ice ages since the ice sheet formed!

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December 3, 2013 3:17 am

Bloke down the pub

Collecting data? That’ll never catch on.

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December 3, 2013 3:20 am

ronald

Looks like tree ring data, but Mann fuckt (sorry)that up so it will not work herr to.

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December 3, 2013 3:50 am

Jeff

“a key factor in making predictions about the future of these massive ice reservoirs and their contribution to sea level rise in a changing climate”…
Why can’t they just cut to the chase and say “please send more money”…
What’s really irritating/frustrating/etc. about this article, which is generally interesting and
readable is that it starts off from a slanted point of view (sea level rise) and taints the
remaining content from there. How refreshing it would be if they would admit the truth
and say “we have no clue about what’s going on here, but we’d like to do more research
on CHANGES in the ice sheet and their effect (s)” (on sea level,etc.). Could be that
it goes the other way,too, perhaps changes in sea level affect the ice? Or what about
changes in the rock or mantle below? What about volcanic or seismic activity?
There are a ton of things this equipment could find out in addition to sea level that
could be of great use (and interest, even if not immediately).
What a pity they can’t just say, let’s do the science…wouldn’t that do justice to the
explorers of old..
Then again, even Apollo et. al. wouldn’t have been funded were it not for the political ramifications of the space program…

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December 3, 2013 4:09 am

Patrick Adelaide

A few years back there was a project to map Artic ice using radar. I think run by a German uni. As I recall, it only lasted one season but had interesting results re ice extent and thickness. Seems like radar is a useful tool for ice mapping purposes. I wonder what happened to that other project?

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December 3, 2013 4:19 am

Katherine

Was the reference to Greenland in the title from NASA’s headline? Kind of misleading, what with the article focusing on Antarctica and the mention of Greenland just a few passing references toward the end. What a fluff piece. They’re obviously trying to justify the program’s existence. They should have highlighted the newly discovered canyon, instead of Dome C Vostok.

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December 3, 2013 4:28 am

LearDog

Striking image, waiting for the micro stratigraphy of the Greenland icecap study. I’m surprised how conforamable the layers are. I imagined that there would be some sort of evidence of significant movement in the past (deformation or significant truncation)
or perhaps evidence of how the ice accumulated (onlap, progradation).
They ought to study from a perspective of sequence stratigraphy – would be fun! 😀

give the prestack seismic (cos this effectively what it is) data to a geophysicist and get the 3D model processed. Then you get loads of events (horizons or reflectors) and can analyse each separately. I agree with LearDog – sequence this Puppy!! Wonder what happened at around 2000m thickness? ago – major dark horizon. Vulcanism or compaction??

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December 3, 2013 4:56 am

Jeff Alberts

And as we all know, CReSIS is just one letter away from CRISIS! Hold on to your wallets!

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December 3, 2013 5:25 am

Pamela Gray

I love this! Fascinating! Reminds me of teasing out the much weaker but non-random auditory brainstem response to sound buried in the cacophony of our larger busy random cortical level brainwaves.

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December 3, 2013 5:28 am

Billy Liar

@ LearDog
The layering in the picture above is relatively undistorted, I believe, because Vostok is a dome; ie a summit. There are very low flow rates of the ice away from the summit hence the lack of distortion.

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December 3, 2013 6:12 am

Paul Coppin

That topo map of Greenland makes it look like Greenland is a impact crater, tectonically squashed… The density layers might correlate with vulcanism on nearby Iceland…

> This is why MCoRDS uses a relatively low frequency—between 120 and 240 MHz.
Wow, the US FM broadcast band only goes up to 108 MHz or so. Conventional radar is in the GHz range.
Cool stuff.

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December 3, 2013 6:22 am

Paul Coppin

Oops, duh. Scratch the speculation about Iceland, seeing as how that’s a Vostok trace. errrrggh.

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December 3, 2013 6:23 am

BioBob

erm – I always read that the bedrock bowl shape was the result of megatons of ice deforming the original surface elevation. Also that once the miles of ice melted away, the elastic nature of bedrock allowed the surface to rise back into it’s previous configuration. But nevermind….

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December 3, 2013 6:50 am

Pittzer

I guess NASA would be interested in this technology to use on future exploration of Jupiter’s icy moons (JIMO). One can only hope.

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December 3, 2013 7:14 am

Doug

Every technical issue discussed has been extensively addressed by the seismic industry. It is clear they need to get Big Oil involved. Oh, the irony!
@leardog—-I see all sorts of interesting disconformities in that image. There is some remarkable documentation of non-anthropogenic climate change in there!

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December 3, 2013 7:16 am

Ed Mr. Jones

Note the DC-3 in the background! First built in the 2nd half of the 1930’s! Still utile and viable. Let’s see a Reel Klimut Sientist (TM) build something like that.

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December 3, 2013 7:21 am

Ed Mr. Jones

Looks like the DC 3 Got a PT-6 Gas turbine power plant upgrade – another exercise in engineering perfection.

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December 3, 2013 7:25 am

kim

Yes, Greenland’s snowcone lies in a bowl, an admission I had to pull like an impacted molar from a CReSIS engineer. They are looking for evidence the ice mass will slip catastrophically off the island and not that it is stable. So, caveat inquisitor.
===========

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December 3, 2013 7:37 am

james griffin

They are desperate to find evidence of AGW…..some scientists will have open minds and be objective…and really enjoy this project. Others will have but one agenda……and we all now what that is.

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December 3, 2013 8:08 am

Jimbo

“Exactly what the future holds remains to be seen,…..”

But it will be worse than we thought. Wait until researchers start cherry picking areas that show any ice thinning and ignore thickening areas.

The Basler BT-67 is a fixed-wing aircraft produced by Basler Turbo Conversions of Oshkosh, Wisconsin. It is built on a retrofitted Douglas DC-3 airframe, with modifications designed to improve the DC-3’s serviceable lifetime. The conversion includes fitting the airframe with Pratt & Whitney Canada PT6A-67R turboprop engines, lengthening the fuselage, strengthening the airframe, upgrading the avionics, and making modifications to the wings’ leading edge and wing tip. http://en.wikipedia.org/wiki/Basler_BT-67

They are truly amazing planes. As the old saw has it: The only replacement for a DC-3 is another DC-3.

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December 3, 2013 8:26 am

Resourceguy

Geology rules…..if anyone will listen.

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December 3, 2013 8:58 am

tty

Paul Coppin says:
“That topo map of Greenland makes it look like Greenland is a impact crater, tectonically squashed… The density layers might correlate with vulcanism on nearby Iceland…”
No. Greenland is unusual since it has escarpments caused bu rifting and oceans opening on both coasts, so it was always rather high at the edges and low in the middle (like southern Africa). Then the weight of the ice has depressed the center even more. It is this bowl-shape that makes the Greenland ice-sheet so uniguely stable. It can’t calve out in any direction. The only way it can disappear is by melting in place.

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December 3, 2013 10:23 am

Mac the Knife

Ed Mr. Jones says:
December 3, 2013 at 7:21 and 7:25 amLooks like the DC 3 Got a PT-6 Gas turbine power plant upgrade – another exercise in engineering perfection.
Ed,
There’s a company called Basler Turbo Conversions up in Oshkosh WI that retrofits DC3s with modern turboprop engines, refurbishes and ‘zero times’ the airframes, adds ‘plugs’ to stretch the fuselage, and provides ski packages for polar operations. The plane shown may be one of their conversions. http://www.baslerturbo.com/
Enjoy!
MtK

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December 3, 2013 11:47 am

george e. smith

“””””…..“a key factor in making predictions about the future of these massive ice reservoirs and their contribution to sea level rise in a changing climate”……..”””””
Would it still be a key factor in making predictions about the future of these massive water reservoirs, and their contribution to sea level fall, in a changing climate ??

I am a little suspicious of the image. There are no faults or fractures in the overlying ice. I recall seeing a radar image from the late 1970s, early 1980s over water in Europe. Several linear features were mapped in several hundred feet of water that corresponded exactly with buried pipelines. This was satellite data. It seems hard to believe that subtle variations in ice layering could be imaged to depths of 3.5 km although I believe the bedrock interface could be located. But what else can a sceptic say?

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December 3, 2013 1:46 pm

Gary Pearse

The bedrock looks too jagged. I wonder why they don’t use seismograph equipment. A slow speed sound transmitter like ice above high speed bedrock should give a fair profile of the bedrock ice interface I would think. There are probably geophysicists here who can say why I might be wrong. My formal education was back when geologists were also geophysicists. I did a Bachelors thesis on profiling of glacial till and Lake Agassiz clay thicknesses over bedrock in Manitoba. We used the first electronic portable “hammer” seismograph. The impulse was imparted by a sledgehammer on a steel plate on the ground that set off a counter – flashing lights in milliseconds; which stopped when a distant geophone received the sound back. Since I have done my share of electromagnetic (EM), magnetometer and scintillometer surveys over promising mineral ground.

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December 3, 2013 1:49 pm

Duster

Steve from Rockwood says:
December 3, 2013 at 1:46 pm
I am a little suspicious of the image. There are no faults or fractures in the overlying ice. I recall seeing a radar image from the late 1970s, early 1980s over water in Europe. Several linear features were mapped in several hundred feet of water that corresponded exactly with buried pipelines. This was satellite data. It seems hard to believe that subtle variations in ice layering could be imaged to depths of 3.5 km although I believe the bedrock interface could be located. But what else can a sceptic say?
There are two potential sources of those lines in the image. One is variation in ice density. The other is interference between radar pulses. Similar images are used in oil exploration all the time to even greater depths and with considerable success, so there’s no need to overwork your scepticism.

Duster says:
December 3, 2013 at 2:09 pmThere are two potential sources of those lines in the image. One is variation in ice density.
That is the main reason: for high accumulation ice sheets, the winter and summer ice shows differences in density which can be counted and are visible on sonar and radar.
For the low accumulation ice sheets that are longer colder and warmer periods, but the deepest ice is too compressed to see any layers.
The layered ice sheet goes back to ~420 kyr. Below that the ice sheet is disturbed because the flow is over the bedrock ridge (left at the figure). Just under the middle of the figure, there is lake Vostok. A schematic view of the Vostok ice core taken years ago and lake Vostok is here:http://www.ldeo.columbia.edu/~mstuding/new_vostok_cartoon_high.jpg

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December 3, 2013 3:16 pm

Steve from Rockwood

Duster says:
December 3, 2013 at 2:09 pm
Steve from Rockwood says:
December 3, 2013 at 1:46 pm
[snip]
There are two potential sources of those lines in the image. One is variation in ice density. The other is interference between radar pulses. Similar images are used in oil exploration all the time to even greater depths and with considerable success, so there’s no need to overwork your scepticism.
—————————————————-
Duster. I’m not an expert in radar but…unlike seismic, radar is an electromagnetic technique that is sensitive to variations in conductivity and dielectric properties. I’m trying hard to believe that annual variations in fresh water ice composition can be so accurately mapped. It looks too good to believe. Just sayin…

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December 3, 2013 6:38 pm

Brian H

Um, that pic is not Greenland, dudes. Vostok is in Antarctica. All you “analyses” are of anal origin.

Steve from Rockwood says:
December 3, 2013 at 6:38 pmradar is an electromagnetic technique that is sensitive to variations in conductivity and dielectric properties.
Besides differences in density, winter and summer snow/ice show differences in conductivity too. That is used to count the yearly layers if too small to be seen with the naked eye. For Vostok the yearly layers are too small anyway and the difference is in colder and less colder periods in the glacial and interglacial periods. The resolution of the age of the ice then is in the decades. The resolution of the gas bubbles is ~600 years.
But the advantage of a small deposit per year (a few mm ice equivalent/year) is that one can go back over 420,000 years before reaching disturbed ice/bedrock.

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